This invention relates generally to the field of composite materials and their methods of fabrication. More specifically, it relates to an improved graphite fabric-reinforced ceramic matrix composite material, and to the method of making it.
Fabric-reinforced composite materials have achieved widespread usage, especially in the aerospace industry, where they are used in the fabrication of structural components of aircraft Most common are fiber-reinforced plastics, wherein polymeric resins are reinforced by a fibrous material, typically graphite.
One problem with such polymer-based materials is their inability to withstand high temperatures. Consequently, for high-temperature applications, fiber-reinforced ceramics have been developed that retain their structural integrity at elevated temperatures. Some typical fiber-reinforced ceramics are disclosed in U.S. Pat. No. 4,284,664 to Rauch, Sr.
With the advent of high-temperature composite materials, there has been an increasing need for suitable materials to fabricate the molds used to form the composite materials into components. Such mold-making materials, ideally, should be capable of resisting high temperatures, and they should be sufficiently durable to withstand prolonged use in the fabrication of components. In addition, such mold-making materials must exhibit good thermal stability (i.e., low coefficient of thermal expansion), low porosity, smooth surface finish, high strength-to-weight ratio, and good vacuum integrity, releasability, and reparability. Also, such materials must be capable of being repeatedly (and easily) heated to process temperatures exceeding 250.degree. C., using internally- or externally-applied heat.
A number of different materials have been employed in attempts to fabricate high-temperature molding tools. While each material has exhibited some advantages, each has also presented shortcomings.
For example, monolithic graphite, while exhibiting low thermal expansion and good high-temperature stability, is fragile, and therefore must be fabricated with thick sections to reduce fracturing. As a result, molding tools made with this material tend to be heavy and difficult to heat. Moreover, such tools exhibit poor vacuum integrity, and they are expensive to fabricate.
Molding tools of steel or other metals have been tried. While exhibiting good high-temperature durability, high strength, and good thermal transfer characteristics, the high coefficient of thermal expansion of metal tools makes them dimensionally inaccurate Such tools are, therefore, unsuited for making close-tolerance components.
Graphite-reinforced polymers have good strength-to-weight ratios, low coefficients of thermal expansion, and good vacuum integrity, but, as previously mentioned, they seriously degrade at high temperatures.
Molding tools made of cast cementitious materials offer good high-temperature durability with low coefficients of thermal expansion. Nevertheless, they have low strength-to-weight ratios and poor thermal transfer characteristics. In addition, they exhibit poor vacuum retention, and their porous structure makes it difficult to obtain good releasability and smooth surface finishes. The addition of metal particulate fillers to such cementitious materials, as taught by U.S. Pat. Nos. 4,482,385 to Satkowski et al. and 4,666,520 to Bright et al., provides greater strength and improved thermal transfer characteristics and vacuum retention. Such tools are, however, still very heavy, and they exhibit a sufficiently high coefficient of thermal expansion to render them unsuitable for fabricating close-tolerance components.
Even fiber-reinforced ceramics themselves have drawbacks as mold-making materials. If such materials are produced without pressing or sintering, as taught by the above-referenced patent to Rauch, Sr., they exhibit high porosity, low dimensional accuracy, and poor surface finish. If such materials are made with pressing and sintering, their fabrication becomes very expensive, due to the extremely high temperatures (approximately 1800.degree. C.) and pressures (approximately 1000 PSI, or seventy atmospheres) involved.
There has thus been a long-felt, but, as yet, unsatisfied need for a mold-making material that meets the mechanical and thermal criteria set forth above, and that lends itself to fabrication into molding tools on a cost-efficient basis.